The present invention relates to the magnetic resonance arts. It finds particular application in conjunction with a radio frequency magnetic resonance coil which is tuned to the resonance frequencies of hydrogen (or other dipoles of interest) for anatomical, angiographic, functional and other medical imaging and will be described with particular reference thereto. It is to be appreciated, however, that the invention will also find application in animal studies, other non-human studies, spectroscopy, multiple dipole studies, very high field studies, multiple B0 field systems, phased-array coil techniques, and the like.
Conventionally, magnetic resonance systems generate a strong, temporally constant static magnetic field, commonly denoted B0, in a free space or bore of a magnet. This main magnetic field polarizes the nuclear spin system of an object. Polarized dipoles of the object then possesses a macroscopic magnetic moment vector preferentially aligned with the direction of the main magnetic field. In a superconducting main annular magnet assembly, the B0 magnetic field is generated along a longitudinal or z-axis of the cylindrical bore. In an open system, the B0 fields extends between a pair of spaced poles.
To generate a magnetic resonance signal, the polarized spin system is excited by applying a radio frequency field B1, with a component perpendicular to the B0 field. The frequency of the magnetic resonance signal is proportional to the gyromagnetic ratio xcex3 of the nuclei of interest times the magnetic field, e.g., 64 MHZ for hydrogen dipoles in a 1.5 Tesla magnetic field. In a transmission mode, the radio frequency coil is pulsed to tip the magnetization of the polarized sample away from the z-axis. As the magnetization precesses around the z-axis, the precessing magnetic moment generates a magnetic resonance signal which is received by the same or another radio frequency coil in a reception mode.
Conventionally, when imaging the torso, a whole body radio frequency coil is used in both transmit and receive modes. By distinction, when imaging the head, neck, shoulders, or other extremity, local coils are often used in conjunction with whole-body coils. Placing the local coil close to the imaged region improves the signal-to-noise ratio and the resolution. In some procedures, local coils are used for both transmission and reception.
One type of coil is known as the xe2x80x9cbirdcagexe2x80x9d coil. See, for example, U.S. Pat. No. 4,692,705 of Hayes. Typically, a birdcage coil has a pair of circular end rings which are bridged by a plurality of equi-spaced straight segments or legs. In a primary mode, currents in the legs are sinusoidally distributed which results in good B1 uniformity across the axis of the coil. B1 uniformity can be further improved by increasing the number of legs in the coil. Capacitors typically interrupt the end rings between adjacent legs and are evenly distributed throughout the two end rings.
As transmit coils are used in higher magnetic fields and at higher power, there is an increased risk of local heating in the end-ring capacitors due to higher voltages and currents. In order to reduce the heating of the end-ring capacitors, additional capacitors are often added to the legs of the coil. While the additional capacitors aid in the lowering of voltage and the reduction of capacitor heating, tuning the coil to desired frequencies becomes problematic due to the cumulative large capacitance values at many positions on the coil. It is difficult to make a significant increment in capacitance with physically small capacitors, which in turn, diminishes the range over which the coil may be tuned.
Conventionally, birdcage resonators are tuned by making a small change to the value of resonating capacitors. This can be achieved by connecting variable capacitors in parallel with fixed value capacitors or by coupling a second nearly resonant circuit to the main coil and adjusting the reactance of the second circuit, typically by means of a variable capacitor. Again, the existence of large values of capacitance at many positions on the coil makes it difficult to adjust capacitance with physically small capacitors. Early birdcage coils typically had eight or sixteen legs with 32 capacitors. Today, many coils contain 20 or 24 legs and 120 or more capacitors. The increase in both capacitance and number of capacitors has made this coil resistant to resonant frequency tuning changes. In response to this problem, arrangements installing conductive foil segments around the end rings of the coil, which form a plurality of variable capacitors, have been employed. In addition, in a 3.0 Tesla B0 magnetic field, hydrogen dipoles have a resonant frequency of 128 MHZ, which poses even further tuning problems.
The present invention contemplates a new and improved radio frequency coil design with enhanced tuning features which overcome the above-referenced problems and others.
In accordance with one aspect of the present invention, a magnetic resonance apparatus is provided. It includes a main magnet which generates a main magnetic field through an examination region. It further includes a tunable RF transmitter coil that is positioned about the examination region such that it excites magnetic resonance dipoles therein. An RF coil assembly receives magnetic resonance signals from the resonating dipoles. A radio frequency transmitter drives the tunable RF transmitter coil and a receiver is connected to the RF coil assembly which receives and demodulates the magnetic resonance signals. The tunable RF transmitter coil includes a pair of end ring conductors which are disposed in parallel planes and are connected by a plurality of spaced leg conductors. The end ring conductors are interrupted by a plurality of reactive elements and the leg conductors each contain at least one reactive element. The reactive element interrupting the leg conductors may be tuned by at least one tuning ring which comprises a non-conductive support cylinder and a plurality of tuning band conductors which extend axially along the non-conductive support cylinder.
In accordance with another aspect of the present invention, a method is provided for tuning a radio frequency coil used in a magnetic resonance apparatus, which coil includes spaced leg conductors electrically connected between two end rings disposed in a parallel spaced-apart relation. The end rings of the coil have at least one capacitance between each adjacent pair of leg conductors and the leg conductors are each interrupted by at least one capacitance. The method for tuning includes positioning a tuning ring containing conductive tuning pads in proximity to the capacitances in the leg conductors and at least one of rotating and translating the tuning ring in relation to the leg conductors and capacitances to adjust a resonant frequency of the coil.
In accordance with another aspect of the present invention, a radio frequency coil for a magnetic resonance apparatus is provided. The radio frequency coil includes a pair of end ring conductors which are connected by a plurality of parallel, spaced leg conductors arranged in a circumferential array. The end ring conductors and leg conductors are interrupted by a plurality of capacitances. The RF coil further includes a segmented conductive tuning ring which is movably positioned parallel to the end rings adjacent the leg conductors in at least one of a capacitively and inductively coupled relationship thereto, such that moving the tuning ring tunes a frequency of the radio frequency coil.
One advantage of the present invention is that it conveniently varies both capacitance and inductance of the RF coil.
Another advantage of the present invention is that it provides a wider tuning range.
Another advantage of the present invention is that a birdcage coil can be tuned to higher frequencies and used in higher magnetic fields and for higher power radio frequency pulses.
Still further advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.